Forums/Z31P/z31-performance-upgrade-forums engine-mechanical-aa/Need help calc'ing CR
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300zxturboftw: (2013-08-06 13:26:53)
So I would need the bore size…88mm (vg 30 .04 over), and what the volume cc is of the VG33 heads and finally the piston stroke length for these motors. Anyone have that on hand?
Careless: (2013-08-06 14:08:06)
83mm stroke. vg33 head should have same (or very close) 55cc combustion chamber volume as the vg30e heads, presuming it's with stock valve sizes (which change the combustion chamber volume by a couple of cc)
Donr1494: (2013-08-06 14:11:52)
Also need piston depth in bore, and head gasket thickness. Lots of numbers to aggregate.
300zxturboftw: (2013-08-06 14:23:40)
1mm oversize valves…thought the vg30e heads were 52cc?
Head gasket thickness 1.13 or 1.29mm (the whole reason I'm trying to calc out CR). Piston depth I have to find out. I think that's really all I'm missing right now is piston values and a confirmation on cylinder head cc.
Careless: (2013-08-06 14:29:30)
i think you mean deck height?
the term "depth" is not to be interchanged or used in place of the measurement for deck height because some motors (many nissans) have pistons that are "proud" of (over) the deck height, and some are "shy" of (under) the deck. And since there is no such thing as "negative depth", the term is called "deck height" and it's given in a positive/negative number depending on where the crown of the piston sits in relation to the deck.
he already knows his piston dish/dome volume, and his head-gasket compressed thickness should be mentioned in the FSM, or he has measured a previous gasket, the headgasket bore size is compatible with .060 over pistons, so it should be somewhere around 88.5 or 89mm, and the other two parameters he asked for… so he has all he needs to calculate CR. not very many numbers to aggregate at all, really.
Careless: (2013-08-06 14:33:23)
why not test the cc's of your heads? are they on the motor yet? all you need is a graduated cylinder/beaker and playdoh.
300zxturboftw: (2013-08-06 14:45:11)
They are at the machine shop and I don't feel like paying them more money. It's also a hassle for me to go over there as I have to take time off work.
Careless: (2013-08-06 14:49:41)
well if your heads were decked or your valves were changed your numbers are going to be different from any ones that someone else gives ya, so… maybe you can have a buddy go get one of your heads for you?
300zxturboftw: (2013-08-06 15:03:36)
So here's where I'm at.
Gasket bore 88.254 mm (88+.01" clearance), 1.29mm thickness (.051), piston 88mm bore (-.004 max for PTW clearance, not a big difference), pin height 32mm, rod length 154.2 and stroke 83 (both stock numbers I'm using, but they are scat rods for VG30 so should be the same). Head was decked .004 thousands which equates to .1mm. Using a 55cc cylinder head volume I'm at 9.29:1 (subtracting .1mm out of the head gasket thickness in the calculator) and at 56cc I'm at 9.15:1. I think I have to go with 1.29 (.051") as the .045 (1.13mm) would be wayyy to high compression.
Found a second link confirming 55cc on 85e heads.
This way I should sit around 9:1 to 9.2:1
Big Slappy Z: (2013-08-06 15:56:52)
Yeah approximately 54.8 cc combustion chamber on 9:1 vg30e.
You're using a vg30 with vg33 heads? what's your setup?
Stock 9.0:1 vg33e combustion chamber is 60.64 cc.
But if you head is decked or the valves are thicker then none of these calculations really matter.
300zxturboftw: (2013-08-06 16:04:02)
Vg30 block with VG33 heads. People say the heads are exactly the same, which means combustion chamber has to be the same too. I calc'd in the amount the heads were decked and subtracted it from gasket thickness. Valves are +1mm, which probably equates to a bump of 1cc-1.5cc's. I think I'm good. Thanks guys.
Big Slappy Z: (2013-08-06 16:17:41)
If they have the same stroke and compression but vg33 has a larger bore, how could they have the same CC volume?
Only way I can think of is bigger piston dome I guess.
300zxturboftw: (2013-08-06 16:36:43)
I'm not sure, that's why I ask. If it's bigger then great my compression will sit a little lower and I will be happy. The question is how much do you think 1mm oversized valves will increase the head cc by.
Careless: (2013-08-06 21:08:02)
piston dome doesn't have anything to do with the cc volume of the head. mainly because the piston is the piston, and the head is the head, but also because if you look at a vg33 piston, there is no dome. there is actually a center dish and two valve reliefs.
theoretically, as long as the quench pads fall within the bore diameter, the head volume doesn't have to change. this could let you use a vg30 head on a vg33 motor. I haven't tried - so I don't know… But i would get my heads back and check the volume because this picture here shows what may be a slight difference in combustion chamber's edge shape so that the chamber gets as close to the firing ring as possible.
[archived image: water_missing.JPG]
now a couple of things. one 9.5:1 is not "waaaaaaaay to high". and 9.2 isn't that far off from 9.5:1. neither is 9:1. if you use good gas or e85 then it prob won't make a huge difference. i'm not sure what your gas is like over there.
and more importantly. lay your gaskets on your head before you go any further because at this point if your combustion chambers overlap the gasket area, then you're fucked and you have 1 of 3 options:
1) weld heads, reshape, and buy super thin gaskets
2) get new heads worked on for vg30
3) get new block worked on for vg33.
surprised you didn't check this BEFORE building the motor
300zxturboftw: (2013-08-06 21:16:18)
According to everyone and there mother, VG33 heads work on a VG30 motor. They look like they are fine and will sit fine as they are pretty identical except for water ports and exhaust studs and throats.
Careless: (2013-08-06 21:29:04)
woops nevermind. the picture posted solves your issues.
the cc should be the same. i thought the lower picture was the vg33 and the chamber was larger to touch the edge of the firing ring, but it looks like the top picture is of the vg33, and you can see the gasket is larger than the chamber itself. so this statement here:
holds true. you should be fine, and it should be same as vg30 cc vol. but i would still check.
also, you're supposed to redrill the steam holes on the vg33 head if using on a vg30.
Big Slappy Z: (2013-08-06 21:39:25)
I know that, but I'm talking about the actual combustion chamber. Since I'm not measuring it myself, or integrating finite points (god forbid) around a model of a vg33 cylinder head, the only way to calculate the volume of the combustion chamber is with known values. That means the combustion chamber volumes are different between vg30 and vg33 (since they are the same stroke & comp & different bore), and the only way the heads are exactly identical is if the pistons have different size center dish/ valve relief, etc., making up for the difference in bore to achieve the same CR.
Like you said earlier, the only realistic and accurate way to calculate the CR of his setup is to have some measured values. Assuming the heads have the exact same CC was the measured assumption he made in order to calculate CR, although I don't know if it is entirely correct. I wouldn't go on "everyone said this though." But who knows…might all work out in the end. Just make sure the assumption is on the conservative side.
300zxturboftw: (2013-08-06 23:26:30)
The small ports by the middle and right combustion chamber? And no you don't redrill them. The MLS gaskets everyone has been using forever blocks them off anyhow. My car ran fine on them blocked off beforehand (vg30 block and heads).
Careless: (2013-08-07 06:17:09)
actualy you could just drill the gasket and the heads to make use of the holes again if you wanted to. if you don't then whatever. it's an option that might help in cooling the heads.
300zxturboftw: (2013-08-07 11:34:11)
I get that, but why drill good vg33 heads that one day might be used on a vg33 block. Those holes are super close to the combustion chamber.
Careless: (2013-08-07 12:10:03)
that's exactly why i would drill them if the clear the firing ring on the new gaskets. could help get rid of some of the heat around the top of the cylinder.
300zxturboftw: (2013-08-07 12:52:09)
I get what youre saying. Just personally its not worth the extra work to me personally. Even if it does take 5 minutes to line up and drill I doubt it makes a huge difference.
Careless: (2013-08-13 05:57:58)
eh no worries. i say anything that makes the engine run cooler is always a good investment, but some people can't be bothered. you have an agenda you'd like to keep and it's been going smoothly for you- so no point in sidestepping it to drill a couple of small holes for a couple of potential degrees of cooling.
300zxturboftw: (2013-08-16 15:43:46)
.035" or .044" piston to head clearance? What do you recommend. I'm thinking .044" to be safe to account for expansion even though my bottom end shouldn't see much with the type of forged metal used.
spddm0n: (2017-05-22 09:52:58)
I just did this recently…you can actually back into it mathematically. You actually just need one take one measurement (piston head volume) and use all the published OEM measurements.
Technically, this is how it plays out…
Total Chamber volume: 53.5cc's, based on a measured piston head volume of 8cc's and compression ratio of 8.9:1. OEM ENGINE SPECIFICATIONS (mm)
(in)
CYLINDER BORE 91.5 PUBLISHED 3.60
STROKE 83.0 PUBLISHED 3.27
GASKET THICKNESS (compressed) (mm) Victor Reinz 1.146 PUBLISHED 0.05
GASKET BORE 92.0 PUBLISHED 3.62
DECK CLEARANCE 0.0 PUBLISHED 0.00
VOLUME (cc)
(cin) (Liters)
CYLINDER HEAD CHAMBER VOLUME (each) 53.5 CALCULATED 3.26 0.053
PISTON HEAD VOLUME 8.0 MEASURED 0.49 0.008
GASKET VOLUME 7.6 CALCULATED 0.46 0.008
SWEPT VOLUME 545.8 PUBLISHED 33.30 0.546
DECK VOLUME 0.0 PUBLISHED 0.00 0.000
TOTAL COMPRESSED (CHAMBER) VOLUME (TDC VOLUME 69.1 CALCULATED 4.22 0.069
TOTAL UNCOMPRESSED VOLUME 614.9 CALCULATED 37.52 0.615
ACTUAL TOTAL ENGINE VOLUME 3689 CALCULATED 225.12 3.689
TOTAL PUBLISHED ENGINE VOLUME (swept volume X 6 cyl's) 3275 PUBLISHED 199.83 3.27
COMPRESSION RATIO 8.90 PUBLISHED
spddm0n: (2017-05-23 22:17:12)
83mm stroke. vg33 head should have same (or very close) 55cc combustion chamber volume as the vg30e heads, presuming it's with stock valve sizes (which change the combustion chamber volume by a couple of cc)
Hey, where are you guys getting 55cc of chamber volume on VG33E heads? Have you measured these, or is this published somewhere? I measured my chamber volume at 49cc's after a 0.005" milling on the heads (I have a second set that was milled by 0.010" and measure about the same volume). I'm trying to determined where the large discrepancy is. I measured the approximate "area" of the chamber opening at the surface (block) to be about 7.3in^2, so if you were to mill the head by .010", you would not reduce the volume by more than 1.2cc. If the OEM standard chamber volume was 55cc's, I would still see 53-54cc's after milling. I'm seeing 49cc's after several repeated measurements. Perhaps my little syringe is that far off??
I also know that Nissan published a large variance on the "nominal height" of the head, so it's hard to say if that comes into play, depending on if this affected the chamber volume, or just the valve cover side of the head.
spddm0n: (2017-05-23 22:23:58)
Also, has anyone measured the piston head volume on a VG33E piston? I measured mine at 8cc's. Is the VG30E much flatter? Did it not have the relief cuts for the valves or the deeper dish? I've never seen the VG30E pistons.
Here is a pic of my VG33E piston.